The present invention generally relates to image forming apparatuses which print a full color image by overlapping color images transferred onto a recording medium by a plurality of electrostatic recording units which are provided with electrophotography printing functions, and more particularly to an image forming apparatus which can detect and correct positions of color images among a plurality of detachable electrostatic recording units.
Conventionally, a color image forming apparatus which employs the electrophotography technique has electrostatic recording units of four colors, namely, black (K), cyan (C), magenta (M) and yellow (Y), disposed in a tandem arrangement along a transport direction of a recording paper. Each of the electrostatic recording units of the four colors forms a latent image on a photoconductive drum by optically scanning the photoconductive drum depending on image data, and develops the latent image into a toner image by a corresponding color toner of a developing unit. The toner images in yellow (Y), magenta (M), cyan (C) and black (K) are successively transferred in this sequence onto the recording paper which is transported at a constant speed, in an overlapping manner by the four electrostatic recording units. Finally, the overlapping toner images on the recording paper are thermally fixed, for example, by a fixing unit.
When the color toner in any of the four electrostatic recording units of yellow (Y), magenta (M), cyan (C) and black (K) run out, it is necessary to replace the entire electrostatic recording unit or a portion of the electrostatic recording unit. For this reason, the electrostatic recording unit has a detachable structure so that it is possible to easily remove and set the electrostatic recording unit with respect to the image forming apparatus in a state where a cover of the image forming apparatus is open.
On the other hand, in order to improve the color printing quality in the color image forming apparatus which has the four electrostatic recording units disposed in the tandem arrangement along the transport direction of the recording paper, it is necessary to improve the color matching accuracy by reducing the positional error of the toner images which are transferred onto the moving recording paper by the four electrostatic recording units. For example, if the resolution in a main scan direction on the recording paper and the resolution in a sub scan direction on the recording paper are respectively set to 600 dpi, the pitch of pixels becomes approximately 42 .mu.m, and the positional error must be suppressed to 42 .mu.m or less. The main scan direction corresponds to a direction perpendicular to the transport direction of the recording paper, and the sub scan direction corresponds to the transport direction of the recording paper.
However, in the conventional color image forming apparatus the four electrostatic recording units disposed in the tandem arrangement along the transport direction of the recording paper, the four electrostatic recording units are detachably provided, and for this reason, the positional error described above is large compared to the case where the electrostatic recording units are fixedly provided. As a result, it was difficult to realize a color matching accuracy having the positional error suppressed to 42 .mu.m of less, due to the mechanical molding or forming accuracy with which parts of the image forming apparatus can be formed and the accuracy with which the parts of the image forming apparatus can be assembled.
A method of eliminating the above described problems associated with the image forming apparatus having the detachable electrostatic recording units was proposed in a Japanese Laid-Open Patent Application No.8-85236, for example. According to this proposed method, resist marks made up of a test pattern are transferred on a transfer belt, and the resist marks are read by a charge coupled device (CCD). The positional error of the resist marks is detected based on an output of the CCD, by comparing detected coordinates of the resist marks with preset absolute reference coordinates of the apparatus. When outputting the image data with respect to a laser scan unit, the image data are corrected based on the detected positional error.
But according to this proposed method, it is necessary to carry out the operation of transferring the resist marks on the transfer belt and detecting the positional error of the resist marks with respect to the absolute coordinates, with respect to each of the four electrostatic recording units of yellow (Y), magenta (M), cyan (C) and black (K). Furthermore, since the CCD is used to detect the resist marks, it takes time to carry out the process of detecting the positional error, and the amount and cost of the required hardware increase.
On the other hand, the conventional image forming apparatus uses the electrostatic recording units which are respectively made up of the laser scan unit which forms the latent image on the photoconductive drum by scanning the photoconductive drum by a laser beam. For this reason, even if the positional error generated by each electrostatic recording unit is large, the scan position of the laser beam can be corrected to a position which is determined by the absolute coordinates in a relatively easy manner in the case of the laser scan unit. However, in order to reduce the size and cost of the image forming apparatus, an electrostatic recording unit which uses a light emitting element array in place of the laser scan unit has recently been proposed. The light emitting element array includes a plurality of extremely small light emitting elements arranged in the main scan direction.
In the electrostatic recording unit which uses the light emitting element array, the beam irradiating position of each light emitting element with respect to the photoconductive drum is determined in a 1:1 relationship by the physical position of each light emitting element. Consequently, unlike the laser scan unit, it is difficult to change the beam irradiating position of the light emitting element for the purpose of correcting the positional error.
It is conceivable to correct the positional error by mechanically adjusting the light emitting element array. However, it would be extremely difficult to make a mechanical adjustment so that the positional error will be suppressed to 42 .mu.m or less. Therefore, in the color image forming apparatus which uses the light emitting element array, a large positional error on the order of 300 .mu.m, for example, is generated, and there was a problem in that a sufficiently high color printing quality cannot be realized by the overlap of the color components.